1. Field of the Invention
This invention relates to a combination of an under voltage lockout {overscore (RESET)} circuit, an electronically erasable programmable read only memory (here in after EEPROM), and a electrical system which are especially useful in space and satellite applications. In digital circuits it is desirable to hold the logic levels at a known state when power is turned on.
2. The Prior Art
In satellites, electrical systems including computers are used for many tasks such as satellite control, telecommunications relay, detection of terrestrial conditions and detection of non-terrestrial conditions as well as for military purposes. In any task where a EEPROM is used, it is desirable to protect the EEPROM from loss of data, or from unintentional programming. EEPROMs are used to store data for operation of computers. Therefore, it is necessary to ensure that the EEPROM data does not become corrupted.
In the case where the power supply voltage to the EEPROM is falling, the EEPROM is particularly vulnerable to loss of data or unintentional programming. As the reference voltage falls, the read, write, address and data lines from a computer may go unstable, thereby providing erroneous information to the EEPROM. Therefore, it is necessary to detect falling supply voltages and to immediately clamp the {overscore (RESET)} terminal of the EEPROM to a very low value (at or near ground) in order to prevent loss of data and unintentional programming.
A EEPROM often used for satellite applications is the Hitachi HN58C1001 series EEPROM, which is described in the specification sheet ADE-203-028F(2) Rev.6.0, Apr. 8, 1997 which is incorporated herein by reference. In the Hitachi 58C1001, when the input voltage Vcc is turned on or off, noise on the control pins generated by the CPU may act as a trigger and turn the EEPROM to program by mistake. In this EEPROM, unintentional programming is prevented by holding the {overscore (RESET)} terminal at a low voltage level (e.g., ground (Vss)) during a time when the supply voltage is cut off. Hitachi teaches that when the {overscore (RESET)} terminal is at a low voltage level, the EEPROM cannot be read or programmed. The unprogrammable state is realized by the CPU""s maintaining the {overscore (RESET)} terminal at cut off (Vss at ground level) when the input voltage (Vcc) is low or off.
To provide a supply voltage responsive {overscore (RESET)} terminal voltage for a EEPROM, useful in a satellite, it is necessary to provide a radiation tolerant circuit which provides for a rapid reduction of {overscore (RESET)} voltage to a low level when the power supply voltage source voltage reaches a predetermined level, and for maintaining the {overscore (RESET)} voltage at a low level at all power supply voltages from 0 to the levels where the {overscore (RESET)} is switched on and off.
The specification sheet for the LinFinity SG1526/B, SG2526/B and SG3526/B regulating pulse width modulators dated April 1990 revision 1.1 dated February 1994, copyright 1994, published by LinFinity Microelectronics Inc. 11861 Western Avenue, Garden Grove, Calif., 92841 is hereby incorporated by reference. In this invention, applicant utilizes the SG1526/B because of it""s superior temperature range, its power supply voltage source voltage, {overscore (RESET)} voltage characteristic curve and its radiation tolerant characteristics which enable it to withstand satellite conditions.
The Unitrode specification sheet is dated June 1993 and describes a product known as UC1526, UC2526, and UC3526. This specification sheet is incorporated herein by reference. The Unitrode device does not have a flat {overscore (RESET)} characteristic under voltage lockout which may be desired for satellite applications as the LinFinity SG1526/B, SG2526/B and SG3526/B circuits, otherwise the under voltage lockout, the soft start, and the digital control port schematic logic appear identical. Still further, the Unitrode products may not possess radiation tolerance which is known to be present in the LinFinity SG 1526/B integrated circuit.
In this specification, applicant will refer to power supply voltage source voltage as the reference voltage sensed by the under voltage lockout. In the LinFinity SG1526/B, Vin (supply voltage) is connected directly to the power supply voltage source voltage, Vin at all times equals power supply voltage source voltage. In Hitachi""s description of the HN58C1001, the input voltage is referred to as Vcc. This voltage is also the same as power supply voltage source voltage which is sensed by the under voltage lockout circuit.
EEPROM""s can lose data when power is transitioning and the control lines on the computer are not in a proper state. This condition can lead to failure in a satellite or other space vehicle. When power supply voltage is increasing, the {overscore (RESET)} line on the EEPROM needs to be held at a low voltage while the power supply voltage source voltage is coming up or increasing. This condition may be met by the power on {overscore (RESET)} capability of the under voltage lockout portion of the LinFinity SG1526/B and the Unitrode regulating pulse width modulator integrated circuits. A more complicated issue occurs when power drops unexpectedly. Keeping the write line high while a 5 volt power drops is not sufficient to guarantee no data loss. The only certain way to eliminate the issue is to hold the EEPROM {overscore (RESET)} line at a low voltage when the power is removed. The solution to the problem requires sensing that the power supply voltage source voltage is going low and producing a low-level voltage signal to the {overscore (RESET)} line of the EEPROM.
This invention uses only a small portion of a LinFinity SG1526/B or a Unitrode UC 1526 regulating pulse width modulator that contains an integrated under voltage lockout {overscore (RESET)} circuit that works properly at low voltages. The LinFinity SG1526/B is available in space level screening, and is compatible with space radiation environments. By supplying both power to the LinFinity SG1526/B Vin pin and the Vref pin (which is usually an output), the desired {overscore (RESET)} characteristics are obtained. The use of the SG1526/B and UC 1526 is possible because the generated {overscore (RESET)} signal is used to provide a {overscore (RESET)} signal to the external circuitry, whereas the intended use was for an internal {overscore (RESET)} signal in the pulse width modulator power convertor application.
The Hitachi HN58C1001 series EEPROM is recognized as a suitable device for use in satellite applications. Applicant has discovered that the LinFinity SG1526/B regulating pulse width modulator integrated circuit is a device which is radiation tolerant, and which includes an under voltage lockout {overscore (RESET)} circuit and output terminal which has the flat {overscore (RESET)} characteristic which provides improved protection of a EEPROM on a satellite. On the other hand, the Unitrode circuits do not have the flat {overscore (RESET)} characteristic under voltage lockout and for this reason may provide less protection to a EEPROM at low power supply voltage source voltage. The SG1526/B provides the necessary reduction of {overscore (RESET)} voltage to a low level and holds {overscore (RESET)} at the low level when power supply voltage source voltage reaches a predetermined low voltage level. The {overscore (RESET)} voltage is maintained at a low voltage level at all times and at all values of power supply voltage source voltage below the turn off/turn on voltage of the SG1526/B under voltage lockout circuit.
Additional objects and advantages of the invention are set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be further realized and obtained by means of the instrumentalities and combinations, particularly pointed out in the claims.